I have power wheels f250 12 volt, I am an HVAC tech so I have motor and electrical background. My question is if you increase the voltage to a motor, will the motor burn out? I would relate it to applying 208v to 115v, motor working with ac motors they burn out quickly. Thank you for your time

Fundamentally your limiters are going to be:
(A) Heat production. 18V vs. 12V could easily produce 2x the waste heat or more, depending on the load. Excessive temperature will decrease the lifetime of the motor (accelerated brush and bearing wear) or can result in direct failure (insulation failure in the windings). Using a larger motor helps -- more thermal mass and dissipation area. Some of the Power Wheels have better airspace around the motors than others as well.
(B) Bearing and brush speed - most brushed motors are not going to last long at 40,000 RPM.

The stock motors are likely fine at 18V, as they're not extremely high power motors and at 18V are still spinning only 23000 RPM, and they only consume ~10-15W at no-load conditions. On the other hand, if you took a 12V motor that started off with higher output it would likely have a short life at 18V. For instance, the Nichibo 8021F motor I referenced in the motor database -runs 23,000 RPM at 12V and pulls 2x the current at no-load. At 18V this would be spinning 34000 RPM.